Industrial automation systems often include a Programmable Logic Controller (PLC) coupled to other devices, such as remote I/O devices, over one or more communication networks. In a factory or other industrial setting, such networks may include dozens of sensors that constantly monitor such things as temperatures, fluid levels, or robotic arm positions. The sensors may be coupled to I/O devices that report back to the PLC many times per second. The PLC may include rules for acting on the changing conditions, such as issuing commands to other devices over the network to perform various operations in the system. The PLC may also monitor and report on the status of various operating parameters and errors detected in the system.
As shown in FIG. 1, a user console 101 is coupled to a programmable logic controller (PLC) 102. Console 101 may comprise one or more processors and memory storing application software and/or a user interface that allows a user or a computer program to configure, alter, control, and monitor the operation of functions in PLC 102 and a network in which the PLC operates. PLC 102 may comprise any of various types of commercially available PLCs, such as the Modicon™ Quantum™ PLC available from Schneider Electric, Inc.
PLC 102 may be coupled to one or more communication controllers 103 and 104 through a backplane interconnection or other means. Each communication controller handles communication with one or more remote I/O devices, such as devices 105, 106, and 107, over one or more networks 108 and 109, for the purpose of monitoring and controlling various devices and sensors in the system.
Industrial Ethernet is one type of network used in industrial automation systems to communicate among nodes. A network such as Industrial Ethernet may be configured into one or more rings that are configured and communicate according to Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP). STP is a network protocol that prevents bridge loops (cycles) and the ensuing broadcast radiation (i.e., packets that are transmitted forever endlessly in the network). It also allows a network to include redundant links to provide automatic backup paths if a link fails, without the danger of creating loops that could lead to redundant broadcast messages.
STP creates a spanning tree within a mesh network of devices and disables any links (blocks ports) between nodes that are not part of the spanning tree, leaving only a single active path between any two network nodes. One node is selected as a root node based on its bridge ID. Other nodes are configured to determine and use the shortest path to the root node. Upon detecting a link failure, the protocol computes and creates a new shortest-path tree structure (e.g., by activating blocked ports), and the network automatically reconfigures itself to work around the failed link or node. STP is standardized as IEEE 802.1D.
RSTP is an improved version of STP, standardized as IEEE 802.1w, that is able to respond to changes more quickly than STP. Other variations and modifications of STP have also been proposed.
It would be desirable to provide improved detection and reporting, particularly at an application level in a PLC, of a detected cable break or other network failure in a network such as an RSTP ring of devices.